U.S. Pat. No. 3,663,667 discloses a process for producing multimetal alloy powders. Thus, multimetal alloy powders are produced by a process wherein an aqueous solution of at least two thermally reducible metallic compounds and water is formed, the solution is atomized into droplets having a droplet size below about 150 microns in a chamber that contains a heated gas whereby discrete solid particles are formed and the particles are thereafter heated in a reducing atmosphere and at temperatures from those sufficient to reduce said metallic compounds at temperatures below the melting point of any of the metals in said alloy.
U.S. Pat. No. 3,909,241 relates to free flowing powders which are produced by feeding agglomerates through a high temperature plasma reactor to cause at least partial melting of the particles and collecting the particles in a cooling chamber containing a protective gaseous atmosphere where the particles are solidified. In this patent the powders are used for plasma coating and the agglomerated raw materials are produced from slurries of metal powders and binders. Both the U.S. Pat. Nos. 3,663,667 and the 3,909,241 patents are assigned to the same assignee as the present invention.
In European patent application No. W08402864 published Aug. 2, 1984, also assigned to the assignee of this invention, there is disclosed a process for making ultra-fine powder by directing a stream of molten droplets at a repellent surface whereby the droplets are broken up and repelled and thereafter solidified as described therein. While there is a tendency for spherical particles to be formed after rebounding, it is stated that the molten portion may form elliptical shaped or elongated particles with rounded ends.
Precious metal based powders heretofore have been produced by gas or water atomization of molten alloys or precipitation from solutions such as in U.S. Pat. No. 3,663,667 issued to the same assignee as the present invention. That patent discloses one method of obtaining solids metal values from a solution. All three processes have some obvious technical drawbacks. Gas atomization can produce a spherical particle morphology, however, yields of fine powder can be quite low as well as potential losses to skull formation in the crucible. Water atomization has the same disadvantage as gas atomization, moreover, it produces an irregular shaped particle which may be undesirable for certain applications. Resulting powder from water atomization usually has a higher oxygen content which may be detrimental in certain material applications. The third process, precipitation from solutions followed by reduction to the metal or metal alloy can be quite attractive from the cost standpoint. Drawbacks are related to the lack of product sphericity and in some instance agglomeration during reduction which lowers the yield of the preferred fine powder of a size below about 20 micrometers.
Fine spherical precious metal based powders such as gold, silver, platinum, palladium, ruthenium, osmium and their alloys are useful in applications such as electronics, electrical contacts and parts, brazing alloys, dental alloys, amalgam alloys and solders. Typically, materials used in microcircuits have a particle size of less than about 20 micrometer as shown in U.S. Pat. No. 4,439,468.
By the term "precious metal based material" it is meant that the precious metal constitutes the major portion of the material thus includes the precious metal per se as well as alloys in which the precious metal is the major constituent, normally above about 50% by weight of the alloy but in any event the precious metal or precious metals are the constituent or constitutents having the largest percentage by weight of the total alloy.
It is believed therefore that a relatively simple process which enables finely divided precious metal and precious metal alloy powders to be hydrometallurgically produced and thermally spheroidized from sources of the individual metals is an advancement in the art.